The present invention relates to a variable light attenuator which is used to continuously attenuate the optical power being transmitted through the optical fiber communication circuit, and especially to the variable light attenuator of high-performance type wherein the harmful reflection light loss is extremely reduced.
A variety of variable light attenuators have been proposed and used to attenuate the optical power passing through the optical fiber communication circuits.
FIG. 6 is a side view of the conventional variable light attenuator, wherein the transmittance continuously varies as a glass plate 3 goes up or down.
In FIG. 6, 1 and 2 are rod lenses arranged along the optical axis of transmission path 4, and 3 is the glass plate whereon a metal thin film is deposited so that the photographic density continuously varies with the position thereof. Glass plate 3 is placed inbetween rod lenses 1 and 2 along transmission path 4 so that the photographic density continuously varies with the position thereof.
When glass plate 3 moves orthogonally to the optical axis (which is indicated by arrow) on an adjusting device (not shown), the light power on transmission path 4 decreases or increases responding to the motion of glass plate 3. At that time, the reflection of the light power at the interfaces between the right-hand surface of rod lens 1 and the left-hand surface of glass plate 3, and between the right-hand surface of glass plate 3 and the left-hand surface of rod lens 2 causes the optical power loss. If a greater percentage of the reflected light power goes back to the light source, the light source becomes unstable.
This type of variable light attenuator sometimes operates unstably because the transmittance of glass plate 3 changes with elapsing of time causing unstable attenuation.
If the distance between the faced edges of a pair of ferrules in an optical connector can be controlled by adjuster, the optical power loss changes as well known by the personnel skilled in the art.
FIG. 7 shows a cross-sectional view of the variable light attenuator wherein the optical power loss can continuously be changed by adjusting the distance between the faced edges of a pair of ferrules.
The connector structure wherein the distance between the faced edges of a pair of ferrules in the optical connector can be controlled by adjuster, as shown in FIG. 7, was proposed by the inventor of the present invention (Japanese Patent Application No. 5849/1990).
Alignment adapter 5 is a cylinder made of a metal, at the center of which through hole 20 is provided to insert ferrules 14 and 15.
Female screw 22 of larger diameter than through hole 20 is provided at the edge in the inner surface of the through hole 20, and the screw 22 is connected with adjusting screw nut 21.
Screws 12 and 13 are provided at the edges on the circumferences of the outer cylinders in alignment adapter 5 and they are coupled with screws 10 and 11 of coupling nuts 8 and 9 in optical connector plugs 6 and 7, respectively.
Key grooves 18 and 19 are provided on screws 12 and 13 so that they can mate with keys 16 and 17 on the outer cylinders of ferrules 14 and 15 in optical connector plugs 6 and 7, respectively.
Compressed coil spring 31 is inserted into the inner of coupling nut 8 and within a gap between coupling nut 8 and flange of ferrule 14, and compressed coil spring 23 is inserted into the interior of the coupling nut 9 and within a gap between coupling nut 9 and flange of ferrule 15. These coil springs 23 and 31 are used to press ferrules 14 and 15 toward the center of the alignment adapter 5. Stop rings 24 and 25 mate with grooves 26 and 27 of ferrules 14 and 15, respectively. Hollow 28 is provided so that adjusting screw nut 21 can rotate around the optical axis of alignment adapter 5.
How to set the attenuation on the variable light attenuator will briefly be described hereafter.
Set adjusting screw nut 21 and then connect optical connector plugs 6 and 7 to alignment adapter 5 as illustrated above. Connect optical fiber 29 to a light source (not shown), and also optical fiber 30 to an optical powermeter (not shown). Then, read the optical power attenuation on the optical powermeter.
Revolve adjusting screw nut 21 a little after detaching optical connector plug 7 from alignment adapter 5, attach optical connector plug 7 to alignment adapter 5, and then read the attenuation by optical powermeter to check if the attenuation is as specified. Repeat the above operations until the specified amount of attenuation can be obtained at designated gap S.
The above operation procedures are too much complicated, and the light power loss which may be caused by the reflection of light at the interfaces between the optical component surfaces and the intermediate air gaps is of 10 to 12 dB in most cases. This greater power loss prohibits this type of variable light attenuator from the use in long-distance, high-capacity communications or in video signal communications.
The objective of the present invention is to provide a new type of variable light attenuator with a reduced light power loss at the interfaces between the optical component surfaces and the intermediate air gaps, wherein attenuation can be set as specified by easy and simple operation procedures.